Section of General Internal Medicine, Boston University School of Medicine, Boston, MA.

Abstract

STUDY OBJECTIVE:

Evidence on variability in emergency medical services use is limited. We obtain national evidence on geographic variation in the use of ambulance transport to the emergency department (ED) among Medicare enrollees and assess the role of health status, socioeconomic status, and provider availability.

METHODS:

We used 2010 Medicare claims data for a random sample of 999,999 enrollees aged 66 years and older, and identified ambulance transport and ED use. The main outcome measures were number of ambulance transports to the ED per 100 person-years (ambulance transport rate) and proportion (percentage) of ED visits by ambulance transport by hospital referral regions.

Notes1. In defining the four quartiles for each of the 3 outcome measures, we used the direct standardization method to estimate age-sex adjusted rate of each outcome measure by hospital referral region (HRR). We then grouped the 306 HRRs into quartiles by the HRR-level age-sex adjusted outcome rate (e.g., ambulance transport rate) and HRR-level enrollee population so that all quartiles have approximately similar enrollee population sizes. To enable adjustment of estimates (and standard errors) for stratified sampling design and weights we implemented direct standardization using Poisson regression models. For ambulance transport rate we estimated a person-level model (N=999,999), and for proportion of ED visits by ambulance we estimated an HRR-cohort level model (N=306*3*2=1,836) where each HRR enrollee population was stratified by age (3 categories) and sex.